1. Field of the Invention
The present invention is in the field of power semiconductors generally and is specifically directed to overvoltage self-protected thyristors.
2. Description of the Prior Art
Typically overvoltage protection of a thyristor employs an avalanche current in the gate region to trigger the thyristor. The avalanching is achieved by etching a deep well, approximately 10 mils, in the gate region, during processing of a silicon wafer, the etching usually occurring after an aluminum diffusion and before a gallium diffusion is carried out. The avalanche voltage is determined by the depth and profile of the etched well.
The use of avalanching for self-protection will succeed or fail depending upon whether the avalanche voltage is less than or more than the edge breakdown voltage of the device.
The use of avalanching necessarily involves some derating of the electrical parameters of the device. Particularly, there is a derating of the forward blocking voltage, V.sub.DRM, along with an attendant increase in forward drop, V.sub.F, for the same V.sub.DRM.
The deep well prior art is discussed in "Thyristors With Overvoltage Self-Protection", J. X. Przybysz and E. S. Schlegel, 1981 IEDM, pgs. 410-413. No patent was ever filed since rights accrue to EPRI, who declined to file.
Two other prior art methods of overvoltage protection are (1) a thinned anode base for controlling V.sub.BO location and voltage level, and (2) using a curved forward blocking junction.
The thin anode base and curved junction technique for achieving overvoltage protection are discussed in "Controlled Thyristor Turn-On For High DI/DT Capability", V. A. K. Temple, 1981 IEDM, pgs. 406-409.
The use of auxiliary thyristors and inhomogeneous or heterogeneous doping of the n-type base region is discussed in "A Thyristor Protected Against di/dt Failure At Breakdown Turn-On", P. Voss, Solid State Electronics, 1974, Vol. 17, pgs. 655-661.
U.S. Pat. No. 4,003,072 teaches curved junctions as a means of overvoltage protection.
"A New Bipolar Transistor-GAT", Hisao Kondo and Yoshinori Yukimoto, IEEE Transactions On Electronic Devices, Vol. Ed. 27 No. 2, February 1980, pgs. 373-379 is a typical example of prior art teachings of a transistor in which the base region has portions extending deeper into the collector region than the remainder of the base region to contact the depletion region.
Application Ser. No. 190,699 filed Sept. 25, 1980 is an example of several applications filed in which the p-type base region of a thyristor has spaced-apart portions extending into the n-type base region to contact the depletion region.
Application Ser. No. 357,106, filed Mar. 3, 1982 teaches providing overvoltage protection in a thyristor by pulsing the center of a gating region of a thyristor with a laser thereby deforming the blocking junction and resulting in a portion of the p-type base extending into the n-type base region.